A tribunal in Nigeria has annulled the election of the president of the senate, David Mark.

Mr Mark was chosen as a senator for Benue state in last April's disputed elections, but now a local tribunal has ordered a re-run there.

Opposition candidates had contested the result amid reports of irregularities.

This latest decision comes only days before another tribunal rules on whether President Umaru Yar'Adua should keep his job.

Last year's national and state elections were widely seen as flawed after international observers dubbed them 'not credible'.

There were allegations of widespread vote-rigging and fraud in the poll - the first in which a civilian leader took over from another following decades of military rule.

Political uncertainty

More than two-thirds of state governorships went to the ruling People's Democratic Party, which also won a large majority in the National Assembly. Mr Yar'Adua is said to have garnered 70% of the vote.

On Tuesday, a special tribunal in the capital, Abuja, is due to rule on whether Mr Yar'Adua's election should be annulled.

A tribunal will rule on Tuesday on the Nigerian president's election

His two main rivals in the election challenged his victory. The case is expected to go to appeal in the Supreme Court.

The David Mark case is seen as creating further political uncertainty because under the constitution, he would take over temporarily as national president should Mr Yar'Adua and the vice-president be forced to step down.

Independent judiciary

Mr Mark is expected to appeal and would remain a senator while that process is underway.

"For now he remains the Senate president until all the legal channels are exhausted," said government spokesman, Cletus Akwaya.

David Mark was in theory at least a very powerful man, says the BBC's Alex Last in Lagos, so the decision to overturn his election is the latest sign that the Nigerian judiciary is increasingly willing to exert an unusual degree of independence.

The Benue tribunal has annulled the victories of all three senators from the state.